1. Field of the Invention
The present invention relates to a recording sheet and, particularly, to an improved color developer composition capable of developing color when contacted with a substantially colorless electron donating organic compound (hereinafter "a color former").
2. Description of the Prior Art
Recording materials utilizing the color forming reaction of color formers with adsorptive or reactive compounds (hereinafter "color developers") which cause a color to develop upon contact with color formers, with specific examples including clay materials, such as acid clay, activated clay, attapulgite, zeolite, bentonite and kaolin; organic acids, such as salicylic acid, tannic acid, gallic acid and phenol compounds; the metal salts of these organic acids; and acidic polymers, such as phenol-formaldehyde condensates, are well known.
The term "color former" as used herein is intended to mean all compounds which become colored through donation of electrons or acceptance of protons, and the term "color developer" as used herein is intended to mean all materials which cause a color to develop through acceptance of electrons or donation of protons.
Specific examples of recording materials utilizing this phenomenon include pressure-sensitive copying papers (e.g., as disclosed in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, and 3,896,255, Japanese Patent Application (OPI) No. 44009/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and so on), heat-sensitive recording papers, electro-thermal recording papers (e.g., as disclosed in Japanese Patent Publication No. 4160/68, U.S. Pat. No. 2,939,009, and so on). In addition, a printing process in which a colored image is obtained by supplying a color former-containing ink to a color developer-coated sheet through a medium, such as a stencil, is disclosed in German Patent Application (OLS) No. 1,939,624. In the utilization of the above-described phenomenon, the color former is contacted with the color developer using an external stimulus of some type, for example, the application of a pressure using a pencil, pen or a typewriter or the like, the application of heat and so on.
The most typical example of these recording materials are pressure-sensitive copying papers. The color former layer of a pressure sensitive copying paper is, in general, prepared by dissolving a color former in a suitable solvent, dispersing the resulting solution into a suitable binder or micro-encapsulating the resulting solution with an appropriate colloid and then coating the material obtained on a support, such as paper, a synthetic resin film, a synthetic resin-coated paper or the like. On the other hand, the color developer is dissolved or dispersed in a medium, such as water, together with a suitable binder and then coated on an appropriate support to obtain a color developer sheet.
In general, a color former and a color developer both may be coated on the same surface of a support, they may be coated separately on different sides of the same support, or the color former may be coated on the surface of a support and the color developer is coated on another support. When the color former solution is microencapsulated, the color former solution is released when a localized pressure is applied thereto by writing with a pencil, a ball-point pen or the like, or by typewriting and, consequently, the color former comes into contact with a color developer to result in coloration corresponding to the localized pressure applied. Thus, recording can be achieved.
A wide variety of color developers are known as described above.
However, when clay materials, namely, activated clay, acid clay and the like, which are currently employed in many cases, are used as a color developer, the resulting coloration product has low resistance to light or water. On the other hand, when a phenol monomer or a phenol-formaldehyde resin is employed as a color developer, the color developer-coated sheet has the disadvantage that the color of the coated surface of the color developer-coated sheet tends to yellow.
Polyvalent metal salts of aromatic carboxylic acids were found to be effective color developers (e.g., as disclosed in U.S. Pat. No. 3,934,070) and were color developers which did not have the above-described disadvantages. Namely, the coloration obtained using one of the polyvalent metal salts of aromatic carboxylic acids as a color developer has the advantages that the color has excellent light resistance, the color is not destroyed on contact with water, the color of the color developer layer does not easily change to yellow when exposed to light for a long time, and so on. However, polyvalent metal salts of aromatic carboxylic acids also have a few weak points as color developers which need to be improved. For instance, when the capsule-coated surface and the color developer-coated surface both are immersed in water in face-to-face contact with each other, a color stain of the capsule-coated surface gradually occurs. The color stain is thought to be caused by a transfer of the polyvalent metal salt of an aromatic carboxylic acid from the color developer-coated surface to the capsule-coated surface through the water due to the fact that such a salt is soluble in water to some extent. This defect inherent in polyvalent metal salts of aromatic carboxylic acids impairs to a marked degree the value of recording materials of this kind as an article of commerce, and it is desired to eliminate this defect. In addition, a color developer-coated surface with a sufficiently high strength cannot be attained when a latex binder along is employed therein, because the polyvalent metal salts of aromatic carboxylic acids are soluble in water to some extent. In order to eliminate this disadvantage, for instance, the combined use of a latex binder and water-soluble binder was tried but was unsuccessful because the water resistance of the color developer-coated surface containing such a combination decreased to such an extent that the colored image obtained was easily stripped off when the colored surface was immersed in water and then was rubbed. This characteristic also is a disadvantage which needs to be eliminated.